Abstract: A circuit/device that can be designed into new telephone subsets as well as connected to existing telephones to prevent unwanted telephone extensions from interfering with the controlling extension. The device is operated by a single push-button located on certain or all extensions. When operated from a certain extension, it disconnects all other extensions that are fitted with the same device until such time that either the push-button at the controlling extension is operated again or the telephone is placed "on-hook." The present device has a very simple circuit located at each extension telephone and communicates over an unused yellow wire found in standard 4-wire telephone circuits.